Tram and oil lamp: what they have in common

1. Step 1. Tram — has part — Pantograph. A current collector.
2. Step 2. Pantograph — is part of — Electric locomotive. Current is collected by a pantograph.
3. Step 3. Electric locomotive — is used in — Metro. The first lines were steam-operated; electric trains since 1890.
4. Step 4. Metro — is made of — Brick. The vaults of the first tunnels are brick.
5. Step 5. Brick — is made of — Clay. Bricks are moulded from clay.
6. Step 6. Clay — is material for — Oil lamp. Later — clay ones.

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